1. Field of the Invention
This invention relates to a process for forming barrier layers at the mold-metal interface of investment molds for casting and directional solidification of superalloys herein.
2. Background of the Invention
Shell molds for precision casting of steel and superalloy parts are, in general, composed of refractory oxide particles bonded together by a silica or phosphate gel. Such molds are generally formed by the "lost wax" process wherein a wax pattern is immersed repeatedly in a liquid slurry of the refractory oxide particles in a silica- or phosphate-bearing binder. Sufficient time is provided between immersions to allow the slurry coat to partially or completely dry on the wax. After a sufficient thickness of ceramic has built up on the wax, the wax is removed by chemical dissolution or melting in a steam autoclave or in a furnace. The mold is then fired, typically at 1000.degree. C. for 1 hour, to give it sufficient strength to withstand the casting process.
Chemical reactions between the mold and the cast metal are a minor problem in conventional casting due to relatively low temperatures and short times that the mold is in contact with the molten metal. However, for the plane front solidification of eutectic superalloys, severe metal-mold reactions frequently occur. These are due to the long contact time (up to 30 hours) of molten metal with the mold, the high temperature (.about. 1800.degree. C.) required in the casting process to enable high growth rates during solidification, and high concentration of reactive elements in the superalloys such as carbon, aluminum, and titanium. In particular, attempts to cast tantalum carbide-reinforced eutectic superalloys with high nickel content in standard shell molds results in such a severe loss of carbon that the tantalum carbide reinforcing phase is absent from the final cast microstructure, producing a useless casting.
The mold-associated cause of this reaction is the silica phase (5-15 Wt%) present in the shell mold. Silica has a small negative free energy of formation and is reduced by the reactive elements in the eutectic superalloys.
With reference to FIG. 1, when NiTaC-13, a monocarbide reinforced superalloy, is cast in mold 10, an example of the prior art, a reaction occurs between the cast metal and the silica phase of the mold. The result of this reaction is the bright phase denoted by the reference numeral 12 which is NiTaC-13 metal penetrated into the mold and reacted with the silica phase of the mold. The resultant casting is defective due to decarburization of the cast alloy and because of poor surface finish.
Other prominent features in FIG. 1 include plastic mounting media 14, coarse backup grains 16 of alumina from the fluidized bed employed for the application of a sand coat between layers of mold materials, pores or voids 18 in the mold structure which result because of material pullout during polishing, or an actual void in the mold structure, and undissolved alumina 20 of one of the flours comprising the material composition.
Other features shown are mullite 22, light grey in color; a silica rich liquid phase 24 (in the mullite 22), dark gray in color; and small grains 26 of flour of alumina material of the face coat. There is no barrier layer present at what may be termed the interface between the mold and the cast metal.
In our copending patent applications, U.S. Ser. Nos. 586,035 and 586,048, now U.S. Pat. No. 3,959,013, we describe how a barrier layer may be formed at the interface between the mold and the cast metal. As illustrated in FIG. 2, the barrier layer 100 is present at the interior wall surfaces of the mold. The interior wall surfaces define the cavity in the mold into which the metal is cast for directional solidification. Coarse grains 102 of alumina from the sand coat of the mold are present. Fine grains 104 of alumina are principally from the alumina flour mixture of the face coating. Bright spots 106 are small metal alloy inclusions in the protective alumina barrier layer 100. Dark area 108 are voids or holes occuring as a result of grain pullouts during polishing of the specimen. Light gray areas 110 are plastic mounting material representative of the porosity in the mold structure after casting and solidification of a superalloy in a mold resulting from the reduction of the silica binder material.
It was our belief that the barrier could only be formed in place by reducing the silica of the mold and obtain a microstructure of the mold in the vicinity of the cavity into which the superalloy is cast which shows a substantial absence of silica between the grains of another refractory oxide comprising the material of the mold. We have now discovered this condition of the mold does not have to exist in order to obtain the barrier layer. A barrier layer can be formed wherein the prior art porous structure is now absent and a substantially solid structure backs up the layer.
It is an object of this invention to provide a new and improved method to form a barrier layer at the mold-metal interface of an investment mold employed for the casting and the directional solidification of a melt of a superalloy therein which overcomes the deficiencies of the prior art.
Another object of this invention is to provide a new and improved method for forming a barrier layer at the mold-metal interface of an investment mold employed for the casting and the directional solidification of a melt of a superalloy therein by the oxidation of at least one element, or constituent, of the superalloy material composition.
A further object of this invention is to provide a new and improved method for forming a barrier layer at the mold-metal interface of an investment mold employed for the casting and the directional solidification of a melt of a superalloy therein, the microstructure of the mold in the vicinity of, and in contact with, the barrier layer exhibits a substantially porous free structure and the material comprising the same contains silica bearing phases therein in contact with the barrier layer.
Other objects of this invention will, in part, be obvious and will, in part, appear hereinafter.